The critical pressure and critical temperature of CO2 are 7.39 MPa and 31.06° C., respectively. When the CO2 is in an environment where the temperature and pressure exceed the critical temperature and the critical pressure, it enters a supercritical state. The supercritical CO2 is a supercritical fluid which is neither a air nor a liquid, and has both characteristics of a large solubility of liquid solute and easy to diffuse and flow of air. It has aroused extensive attention and is being applied to various fields due to its characters of cheap, easy to gain, non-toxic, pollution-free and uninflammable.
With the development of exploration technology of unconventional oil-gas resources and underground storage technology, the supercritical CO2 has been widely applied. For example, the coal-bed methane is exploited by injecting the supercritical CO2, and a supercritical CO2 cannon is used for deep-hole blasting and permeability increasing in exploitation of the oil-gas resources. For another example, injecting liquid CO2 into unminable coal layer by technology of underground CO2 storage can not only sequestrate CO2, but also separate gas from coal body so as to increase gas production. For yet another example, CO2 can be injected into an abandoned mine chamber to seal the mine. As the storage depth of CO2 increases, the pressure and temperature will also increase accordingly. It is easy to reach the critical pressure and temperature of CO2. In order to better realize application and development of the supercritical CO2, it is very important to fully understand and master the mechanical properties of the rock mass under the condition of the supercritical CO2. For CO2 sequestration, it is of great significance for the preparation and design of the underground sequestration scheme, instruction of on-site construction and later maintenance to understand of creepage, diffusion and erosion reaction of the rock mass in a multifield coupled environment of stress-temperature-supercritical CO2 fluid and under a long-term thermo-mechanical coupling effects.
In current study, relevant seepage diffusion theory and numerical simulation are difficult to accurately predict the creepage, diffusion and erosion of the rock mass in the supercritical CO2 environment. At present, the laboratory test is still a usual means. For example, a test of the creepage, diffusion and erosion reaction of the rock mass is conducted by immersing a cylindrical specimen of rock mass into the supercritical CO2 environment for a preset time beforehand, then removing the specimen to a triaxial experimental machine and applying an axial load on the specimen, and at last applying a confining pressure on the specimen by injecting oil. However, the specimen has been out of the supercritical CO2 environment when the test is conducted, resulting a relatively larger error of the test result.